The present invention relates to methods and apparatus for inducing plasma in low pressure plasma systems, which are typically used in semiconductor fabrication. More specifically, the invention relates to methods and apparatus for variable control of the plasma generating element to achieve combinations of inductive and/or capacitive coupling.
Plasma-enhanced semiconductor processes for etching, oxidation, anodization, chemical vapor deposition (CVD), or the like are known.
For illustration purposes, FIG. 1 shows a chemical etch reactor 100, representing a plasma generating system which utilizes an inductive coil for plasma generation. Reactor 100 includes coil system 102 and chamber 124. Coil system 102 includes a coil element 106, which is biased by radio frequency generator 110 to act as an electrode. Coil element 106 is coupled to a matching circuit 108 for matching the impedance of coil element 106 to that of radio frequency generator 110. The matching of the impedances permits radio frequency generator 110 to efficiently deliver power to coil element 106. To provide a path to ground, the chamber wall of chamber 124 is typically grounded. Alternatively, the ground path may be provided through the lower electrode, e.g., a chuck 128 of FIG. 1, when the plasma is confined.
Within chamber 124, there typically exists a vacuum. A shower head 126 is disposed above a chuck 128 and wafer 134, which is supported by chuck 128. Chuck 128 acts as a second electrode and is preferably biased by its independent radio frequency circuit 120 via a matching network 122. It should be borne in mind that the components of FIG. 1, as well as of other figures herein, are shown only representatively for ease of illustration and to facilitate discussion. In actuality, coil element 106 and match 108 are typically disposed proximate to chamber 124 while RF generator 110 may be placed in any reasonable location.
Shower head 126 represents the apparatus for dispensing deposition materials onto wafer 134. Shower head 126 preferably includes a plurality of holes for releasing gaseous source materials (typically around the periphery edge of shower head 126) into the RF-induced plasma region between itself and wafer 134 during operation. In one embodiment, shower head 126 is made of quartz although it may also be made of other suitable materials and may be left either electrically floating or grounded.
In the prior art, there exists capacitively coupled plasma systems. It has been discovered, however, that inductively coupled plasma generates higher plasma density, which is more suitable for certain low pressure processes. In the prior art, the relative phases at first coil end 130 and second coil end 132 of coil system 102 is a function of the electrical length of the coil and the operating frequency and is consequently relatively fixed.
However, a plasma generating system that is preset to couple its plasma either inductively or capacitively is inherently limiting. Modern fabrication processes demand flexibility on the part of the equipment that are used to fabricate semiconductor circuits. Consequently, there has been efforts to provide for plasma generating systems that can be configured, in a flexible manner, as either an inductive system, a capacitive system, or one that provides for a combination of both inductive and capacitive coupling.
By way of example, there exists in the prior art a control circuit which utilizes four capacitors for producing either inductively coupled plasma or capacitively coupled plasma. This prior art control circuit, which is essentially analog in character, controls the coupling of the coil by varying the capacitance of one or more capacitors.
Although the aforementioned control scheme has some advantages, it nevertheless represents an electromechanical approach, which results in many attendant disadvantages. For example, it is difficult to set up the capacitors in the prior art analog control circuit because the setup parameters depend on the specific measurements pertaining to a particular reactor.
Further, the prior art electromechanical approach to providing the desired combination of inductive/capacitive coupling is static and is therefore difficult to change to accommodate, in a flexible and simple manner, applications that demand different combinations of inductive and/or capacitive coupling. Most significantly, it is difficult to vary, as a function of time, combinations of inductive and capacitive coupling using the prior art electromechanical approach.
In view of the foregoing, what is desired is new apparatus and methods for achieving, in a flexible and simple manner, variable combinations of inductive and/or capacitive coupling in a plasma generating system.